Genistein and daidzein are estrogenic isoflavones found in soybeans. Although they are much less potent than estradiol, infants fed soy formula have serum concentrations that are up to 4 orders of magnitude higher than estradiol. We have been in the field with a longitudinal study of infants up to 9 months old, looking in detail at the growth and development of their estrogen-responsive tissue as well as sex hormones and gonadotropins. Enrolllment of the 270 children will be complete by fall, 2013.